Differentiating acute coronary syndrome (ACS) from noncardiac chest pain is the primary diagnostic challenge for clinicians in the emergency department. Studies of myocardial perfusion imaging have demonstrated important information for risk- stratifying stable post-ACS patients. However, this method is of limited value in patients with prior history of coronary heart disease (CAD), since these patients will often have abnormal resting perfusion patterns precluding the ability to differentiate old infarcts from new ischemic events. 99m Tc-glucarate does not detect old heart attacks, and thus, should provide an advantage in the specificity for imaging suspected ACS patients with previous CAD. 99m Tc-glucarate has been in development as a radiopharmaceutical diagnostic imaging agent for the diagnosis of acute myocardial infarct (AMI) for more than a decade. Previously the agent was thought to be specific for necrotic myocardium or MI. However, more recent preclinical studies suggested that 99m Tc-glucarate uptake is increased in the setting of ischemia, even in the absence of necrosis. This expanded specificity has led to the hypothesis that this imaging agent could be used to accurately assess the presence of ACS in the chest-pain patient with equivocal diagnosis. The broad aim of the proposed phase II clinical trial is to demonstrate that the nuclear imaging product candidate, 99mTc-glucarate, can be used in the diagnosis of ACS in chest pain patients with prior history of coronary heart disease in the setting of the emergency department. The accuracy of diagnosis of ACS in the emergency department with 99mTc-glucarate will be obtained by comparison to the clinical discharge diagnosis. Specific aims of the proposal are: 1. Conduct a multi-site clinical trial of cardiac imaging with 99mTc-glucarate in 100 chest pain patients suspected of ACS, but with no obvious signs of MI and having prior coronary artery disease. 2. Determine the accuracy of 99m Tc-glucarate imaging compared to standard clinical discharge diagnosis. Results from this research could provide a technological advancement in definitive diagnosis of acute coronary syndrome in patients with know coronary heart disease. This should not only lead to early diagnosis and triage of these patients but also should lead to better health care resource management.